The present description references the following documents. The documents are to be referenced by their document numbers.    [Document 1] WO 98/21691    [Document 2] WO 00/36555
When a plurality of responders exist in an effective radio wave area of the interrogator, it is necessary to identify response signals from the responders. Document 1 discloses a technique for preventing cross talk between the responders.
The technique disclosed in Document 1 is as follows. An interrogation signal from the interrogator is received and the responder transmits a predetermined number of bits. The interrogator receives the predetermined number of bits transmitted from the responder and returns them to the responder. When the number of bits returned is identical to the number of bits transmitted, the responder transmits a predetermined number of bits subsequent to the bits already transmitted and repeats the same process. When the number of bits returned is not identical to the number of bits transmitted, the responder does not participate in the identification process until the next interrogation signal is received. By repeating this process, finally, only one responder makes identify its identification number. By repeating this recognition process until no unprocessed responder exists, identification of the responders is complete.
In Document 1, transmission and reception to/from the interrogator are repeated in bit unit of a predetermined number and accordingly, this technique requires a logical circuit for switching between transmission and reception and control of a memory address counter requiring various commands (interrogation signal, reception bit return signal, identification failure report signal, identification completion report signal), a plenty of operation stages accompanying the commands, a flip-flop representing the state transition, and a data comparison circuit.
Document 2 discloses a technique in which a responder having a memory for storing an identification number in accordance with the clock pulse from the interrogator transmits the identification number. In Document 2, the responder as RFID transmits an identification number in accordance with the clock pulse from the interrogator, thereby excluding a command in communication and simplifying the transmission/reception method.